Pendulous Integrating Gyro Accelerometers (PIGAs) and Specific Force Integrating Receivers (SFIRs) are very high accuracy accelerometers primarily used in strategic missile guidance systems. A pendulous accelerometer is based on the principle that an acceleration that displaces a suspended mass can be determined from the angle of displacement. These accelerometers have bias and scale factor performance that are one to two orders of magnitude more accurate than conventional navigation-grade accelerometers such as Northrop Grumman's A4 and SiAc accelerometers. PlGAs and SFIRs, on the other hand, are two orders of magnitude more expensive and have lower reliability, and therefore have high life cycle cost. Alternate, lower cost accelerometers such as vibrating beam accelerometers (VBAs) with improved reliability have been under development as replacements for PlGAs and SFIRs for more than two decades. While a considerable investment continues to be made in these alternate strategic-grade accelerometer technologies, none have achieved the technical readiness level to be deployed.
A variety of Helium-Neon laser accelerometers, based on laser gyroscope technology, in which the internal cavity length, and therefore the frequency of the laser, is a function of acceleration have also been proposed in the past as replacements for PlGAs and SFIRs. In general, cavity length based laser accelerometers measure the full acceleration range with proof mass displacements of less than one wavelength to avoid mode hopping. For a 633 nm laser wavelength the full range displacement of the cavity length is less than 0.32 μm. This displacement corresponds to a full range acceleration of 20 times the acceleration of gravity. The dynamic range of the accelerometer is required to be greater that 107. For the accelerometer to have bias stability and repeatability of less than 2 μg the cavity length must be stable and repeatable over temperature to less than 3×10−14 meters or 0.00003 Å. If the laser body is fabricated from a material such as Zerodur, which has a coefficient of thermal expansion between 0.02 and 0.1 ppm/° C. and the cavity lengths are 10 cm; then the thermal gradient between the two cavities, used to provide common mode rejection of ambient temperature change, would need to be less than 1.5×10−5 to 3×10−6° C. This extreme sensitivity of the accelerometer bias to thermal gradients is one of the principal reasons for prior laser accelerometers not meeting the bias stability and repeatability required for strategic applications.